word choice - "Forty foot" or "forty feet"?

Is it correct to say

"This is a forty foot drop"

or

"That is a forty foot telescope"

Shouldn't it be "forty feet"?

And what if it is

"This is a thirty nine foot drop"

instead?

Are both correct? Does it depend on the context? How does it work?

Answer

The forty-foot drop is correct.

Compare this:

1) He's a 10-year old.

2) He's 10 years old.

In the first example, a 10-year old stands as a noun (notice the article preceding it).
In the second example, it's not a noun, therefore no article.

Therefore:

1) It's a 40-foot drop.

2) This drop is 40 feet in height.